Optical aberrations are defined as deviations from ideal wavefront sphericity or deviations from the homocentricity of a ray bundle. Aberrations are inherent in any optical system composed of spherical surfaces. Aberration polynomials are introduced by decomposing an aberrated wavefront into a Taylor series. This decomposition reveals that low-order wavefront aberrations–namely, defocus, coma, astigmatism, and spherical aberration, represented by Seidel polynomials depending on pupil coordinates and field angle—have the highest magnitude. The correction of these low-order aberrations is of primary importance and can be achieved through the proper design of optical systems utilizing multiple refractive and/or reflective surfaces. Zernike polynomials are introduced as an orthogonal set for representing wavefront error across the pupil. The concepts of aplanatic points on spherical surfaces and the use of aspheric surfaces described by conic sections (both refractive and reflective) are presented. Glass dispersion is the primary source of chromatic aberration, which can be reduced by using appropriate combinations of different glass types in the optical design. Finally, concepts of point spread function (PSF) and the modulation transfer function (MTF) are introduced for proper analysis of aberrations influence on the image quality.